Closes gh-23453

See gh-21778

This commit introduces Kotlin code snippets, for now
in the core reference documentation. Other sections
will follow, as well as improvements like global
language switch.

See gh-21778

See gh-23451

Closes gh-23450

Due to recent changes, the (bean instanceof Aware) check is now
superfluous.

See gh-23445

Closes gh-23445

This commit redeclares default methods in AbstractTestExecutionListener
in order to make them inlinable.

See gh-23435

This commit avoids unnecessary processing in the implementation of
postProcessBeforeInitialization() in ApplicationContextAwareProcessor
by immediately returning the supplied bean if it does not meet the
requirements for further processing.

Closes gh-23435

Closes gh-23436

This commit changes {@code &#42;&#47;&#42;} to <code>&#42;&#47;&#42;</code>
so that "*/*" is rendered correctly.

Closes gh-23439

Prior to this commit, the corresponding documentation was misleading.

Closes gh-23428

* pr/23437:
  Fix typo

Closes gh-23437

See gh-23437

Prior to this commit, StopWatch used System.currentTimeMillis() to
track and report running time in milliseconds.

This commit updates the internals of StopWatch to use System.nanoTime()
instead of System.currentTimeMillis(). Consequently, running time is
now tracked and reported in nanoseconds; however, users still have the
option to retrieve running time in milliseconds or seconds.

Closes gh-23235

This commit avoids the use of the deprecated
GenericTypeResolver.resolveParameterType() method in tests in order to
avoid warnings in the Gradle build.

Closes gh-23391

Since arbitrary levels of proxies do not occur, this commit replaces
the `while` loop in SerializableTypeWrapper.unwrap() with a simple
`if` statement.

Closes gh-23415

Since Java 8, putIfAbsent() is a standard method in java.util.Map. We
therefore no longer need the custom implementation that overrides the
standard implementation in HashMap.

Prior to this commit, AnnotationAttributes#assertNotException checked if
the attribute value was an instance of Exception. Although this was
typically sufficient, the scope was not always broad enough -- for
example, if AnnotationReadingVisitorUtils#convertClassValues stored a
Throwable in the map (such as a LinkageError).

This commit fixes this by checking for an instance of Throwable in
AnnotationAttributes#assertNotException.

Closes gh-23424

Closes gh-23420

Optimize section headings for dispaly in left hand navigation.

Closes gh-23401